Generally, a polyimide film is formed from a polyimide resin. Such a polyimide resin is a highly heat-resistant resin prepared by subjecting an aromatic dianhydride and an aromatic diamine or an aromatic diisocyanate to solution polymerization, thus preparing a polyamic acid derivative, which is then subjected to ring-closing dehydration at a high temperature so as to be imidized. Since a polyimide resin is a very highly heat-resistant resin, which is insoluble and infusible, and is superior in terms of thermal oxidation resistance, heat resistance, radiation resistance, low-temperature characteristics, chemical resistance and the like, it has been utilized in a variety of fields including those of advanced heat-resistant materials, such as automotive materials, aircraft materials, spacecraft materials, etc., and electronic materials such as insulation coating materials, insulating layers, semiconductors, electrode-protecting films for TFT-LCDs, substrates for flexible printed wiring boards, optical communication materials, protective layers for solar cells, etc. Recently, such a resin is employed for display materials such as optical fibers or liquid crystal alignment layers, and is also used for transparent electrode films, either in a manner in which it is contained along with a conductive filler in the films or in a manner in which it is applied on the surface thereof.
However, a polyimide resin is brown- or yellow-colored, attributable to its high aromatic ring density, and thus has low transmittance in the visible light range. Additionally, it takes on a yellowish color, which decreases light transmittance and increases birefringence, making it difficult to utilize it for optical members. Also, when improving the optical properties thereof, a lower Tg (glass transition temperature) than that of conventional yellow-colored polyimide films may result, making it difficult to utilize it in fields requiring high temperatures of 300° C. or more.
With the goal of overcoming such problems, attempts have been made to purify monomers and solvents to high purity before polymerization, but to date the improvements in transmittance have not been significant. Furthermore, the use of a monomer having a rigid structure in order to realize thermal stability may remarkably deteriorate transmittance or may increase yellowness.
With regard to conventional techniques pertaining to polyimide, U.S. Pat. No. 5,053,480 discloses the use of an aliphatic cyclic dianhydride component in lieu of aromatic dianhydride. Although the prepared solution or film is improved in transparency and color compared to when using the purification method, the increase in transmittance is limited, and thus high transmittance cannot be realized, and moreover, deteriorated thermal and mechanical properties may result.
Furthermore, U.S. Pat. Nos. 4,595,548, 4,603,061, 4,645,824, 4,895,972, 5,218,083, 5,093,453, 5,218,077, 5,367,046, 5,338,826, 5,986,036 and 6,232,428 and Korean Patent Application Publication No. 2003-0009437 disclose a novel polyimide having improved transmittance and color transparency in the range within which thermal properties are not significantly deteriorated, using aromatic dianhydride and aromatic diamine monomers, having a linker such as —O—, —SO2—, CH2—, etc., a bent structure due to the connection to an m-position rather than a p-position, or a substituent such as —CF3, etc., but the above polyimide is unsuitable for use in display devices, such as OLEDs, TFT-LCDs, flexible displays, and the like, in terms of mechanical properties, heat resistance, and birefringence.
Meanwhile, these days thorough research into a variety of plastic materials is ongoing as alterative materials to glass substrates in order to realize flexible displays. In particular, polyimide film is receiving great attention because it enables the formation of a curved surface and is also able to achieve properties of thinness, light weight and low brittleness. In order to apply the polyimide material to display processing, there is a need to ensure high thermal stability and a low coefficient of thermal expansion for dimensional stability so as to be capable of withstanding the processing conditions, and also to maintain properties of colorlessness and transparency.